Plugging performance and mechanism of an OBDF oil-absorbing resin (MMA-SMA-St) plugging agent

To treat the issue of increased resource wastage due to the higher plugging tendencies of oil-based drilling fluids (OBDF) relative to water-based drilling fluids, this study synthesized a ternary composite oil-absorbing resin and optimized its synthesis parameters. The influence of temperature variations on the resin's oil absorption capacity was assessed. Techniques such as infrared spectroscopy, scanning electron microscopy, TGA-DSC measurements, crosslinking degree analysis, contact angle analysis, X-ray photoelectron spectrometry analysis and examination of the resin's plugging mechanism were employed to investigate its molecular structure, oil absorption properties, and plugging efficiency. Additionally, the impact of various synthesis conditions on the oil absorption expansion rate of the oil-absorbing resin was examined. The findings indicate that the resin developed in this research maintains robust oil absorption capabilities at 160 °C, exhibiting an oil absorption expansion rate of 12.5 g g−1. At this temperature, the composite resin particles effectively sealed leaks of widths 0.25, 0.5, and 0.75 μm. Comparative analysis revealed that adding 3% of these resin particles to OBDF significantly enhanced the sealing of fractures. Remarkably, at 160 °C, OBDF amended with resin particles managed to completely seal fractures measuring 0.25 μm. The novelty of this study is attributed to the utilization of styrene for enhancing the resin's rigidity, coupled with the application of octadecyl methacrylate, which contains long-chain alkyl groups, to optimize the oil absorption and expansion characteristics of the oil-absorbing resin.


Introduction
2][3] Consequently, the demand for drilling uids in the oil and gas development process has risen. 4Drilling uid, a critical component of natural gas and oil extraction, serves to carry and suspend rock particles, stabilize wellbore walls, and balance formation pressure.It includes a variety of circulating uids used across a broad spectrum of drilling operations in the oil and gas industry. 5,6During drilling, fragile and complex shale rock formations are susceptible to wellbore leakage, potentially resulting in issues such as sticking, blowouts, and thereby leading to signicant economic losses and safety hazards, ultimately impairing drilling efficiency. 7esearch shows that well leakage is responsible for 20% to 25% of all drilling projects, globally, leading to annual economic losses up to $9 billion in the petroleum industry worldwide. 8dressing well leakage at drilling sites is, therefore, an imperative challenge in the drilling uid eld.
Currently, the majority of drilling uids used in oil and gas eld drilling engineering are categorized into water-based drilling uid (WBDF), [9][10][11] OBDF [12][13][14][15] and synthetic-based drilling uid. 16,17Among these, OBDF is oen the preferred option for ultra-deep and complex wells due to its high-temperature resistance, superior lubrication, and enhanced wellbore stability.However, as OBDF systems continue to evolve, the issue of well leakage has become increasingly evident.On the one hand, the predominantly hydrophilic nature of the original rock surfaces within formations and the use of wetting agents in OBDF transform these surfaces from hydrophilic to oleophilic.Conversely, the requirement for oil-based drilling uid plugging materials to exhibit excellent dispersibility in the oil phase necessitates that these materials possess an abundance of lipophilic functional groups.Consequently, this characteristic prevents the plugging material from forming non-covalent interactions, such as ionic and hydrogen bonds, with the oleophilic rock wall.During drilling, capillary forces can cause the wellbore wall rocks to adsorb both water and oil phases, enlarging the gap between fractured rock layers and thus heightening the risk of well leakage.Additionally, the effective lubrication provided by OBDF facilitates its entry into fractures, potentially inducing hydraulic fracturing and consequent well leakage. 18Consequently, the exploration of OBDF-compatible plugging materials has emerged as

RSC Advances
PAPER a prominent research topic.Currently, over a hundred types of plugging materials are utilized at major drilling sites, yet the vast majority are designed for WBDF, with only a limited number being compatible with OBDF. 19,20As a result, in the event of well leakage, technicians oen resort to WBDF-appropriate plugging materials, which tend to yield suboptimal sealing outcomes.In summary, identifying a plugging material that exhibits strong compatibility with OBDF represents a signicant area of interest within the eld.
2][23] Among these, resin-based materials have attracted extensive research interest owing to their excellent compatibility with OBDF.For instance, Li et al. 14 developed butadiene styrene/nano-SiO 2 (SBR/SiO 2 ) composites through a continuous emulsion polymerization process.Their ndings revealed that the integration of silica and styrene not only imparts signicant pressure resistance to the composites but also enables SBR/SiO 2 to penetrate shale formations where leakage occurs, substantially mitigating uid intrusion.Additionally, Du 30 synthesized nanometer polystyrene dodecyl acrylate (PSL) oil absorption resin using styrene (St) and lauryl acrylate (LMA) via a soap-free emulsion polymerization method.This process yielded a product with remarkable dispersion stability and oil absorption expansion capabilities, primarily attributed to the exible macromolecular chains of dodecyl acrylate, which undergo expansion upon oil absorption, facilitating deformation and expansion.Nonetheless, despite these advances, the application of the aforementioned resin-based plugging agents in OBDF systems on-site is limited by high production costs and a lack of universality, hindering optimal compatibility.However, the above research results used as plugging agents, didn't discuss the contradiction between the dispersibility of plugging agents and their staying capacity in the loss channel.In this study, the plugging agent prepared by us can be dispersed in oil-based drilling uids due to its lipophilicity.Additionally, it can also be seen that the plugging agent has certain staying capacity in the plugging experiment.Therefore, the resin prepared in this study is of great signicance for the plugging problems encountered during oileld development.
In this paper, the study has developed a novel hightemperature-resistant suspension polymerization resin (MMA-SMA-St) tailored for OBDF applications.The resin is synthesized using methyl methacrylate (MMA), stearyl methacrylate (SMA), and styrene (St) as monomers.Incorporating methyl methacrylate and styrene addresses the issue of exible crosslinking instability, which is attributed to the lengthy carbon chain present in stearyl methacrylate.Furthermore, the employment of suspension polymerization offers a practical framework for the eld application of this plugging material.

Synthesis of oil absorbing expansion resin
A volume of 120 mL deionized water was transferred into a 250 mL four-neck ask and mixed with PVA.This mixture was stirred at room temperature until the PVA was fully swollen, then heated to 80 °C and stirred for 30 minutes to ensure complete dissolution of PVA, followed by cooling to 50 °C.Monomers MMA and SMA, cross-linker MBA (0.6 g), and poreforming agent EAC (4.5 g) were combined and stirred for 15 minutes, yielding solution A. Separately, the rigid monomer St and initiator BPO were stirred together for 15 minutes, forming solution B. Both solutions A and B were then stirred together for 15 minutes.Solution A was added to the pre-prepared PVA solution under a nitrogen atmosphere.Aer stirring the mixture for 30 minutes, the temperature was increased to 85 °C, and solution B was added.The reaction mixture was stirred continuously for 8 hours, then cooled to room temperature, and ltered to collect transparent resin microspheres of irregular sizes.The microspheres were washed thrice with hot deionized water and ethanol, then dried in a vacuum oven at 80 °C for 24 hours.

Oil absorption and expansion performance
The oil absorption and expansion rate of the resin are crucial for the formulation of oil-based drilling uid systems and the prediction of their plugging efficacy.The oil absorbency (W) of the product was quantied using a gravimetric method.A preweighed 1 g sample of the product was enclosed in a lter bag and immersed in oil at room temperature.For complete oil absorption, a duration of 24 hours was required.Subsequently, the lter bag containing the product was extracted from the oil and permitted to drain for one minute.Immediately aer draining, the product was removed from the lter bag and weighed.Oil absorbency was calculated using the equation below: The oil absorbency is calculated using the formula wherein G 1 and G 2 represent the weights of the resin before and aer oil absorption, respectively.Unless otherwise specied, "oil absorbency" refers to the total oil absorbency. 24

Preparation of OBDF
As noted by Shen et al. 25 taking into account both the stability and the cost of the OBDF, 3# white oil was chosen as the base oil with an oil-water ratio of 80 : 20.The detailed composition of the drilling uid is presented in Table 1.In this context, the amide group within the non-ionic emulsier, namely fatty acid polyamide, serves as a hydrophobic entity oriented towards the oil phase, whereas the amino group functions as a hydrophilic entity directed towards the water phase.This conguration effectively reduces the free energy at the oil-water interface, thereby enhancing the stability of the drilling uid mud.Organic clay, characterized as an oleophilic material, is derived from hydrophilic bentonite modied with dodecyltrimethylammonium bromide (DTAB).To negate the effects of ltrate reducers on the ltration rate, the system is formulated without the addition of any ltrate reducer.

Evaluation of drilling uid performance
Plugging experiment.To accurately assess the sealing efficacy of plugging materials on fractures, the study utilized a custombuilt simulation fracture apparatus in conjunction with a hightemperature and high-pressure (HTHP) ltration instrument (GGS42, Qingdao Hengtaida Electromechanical Equipment Co., Ltd.) for evaluation.The process began with the assembly of a micro fracture simulation rigid seam board, consisting of two semi-cylindrical sections with an external diameter of 54.3 mm.The fracture width was set by inserting aluminum foil strips of varying thickness (20-1000 mm), adjusted to specic widths (x = 0.25 mm, 0.5 mm, 0.75 mm), and secured with nitrile rubber seals.The assembled simulation fracture plate was then placed at the lower end of the HTHP ltration instrument's cylinder, followed by the addition of the pre-mixed drilling uid slurry into the device.The experiment was conducted by setting the HTHP ltration instrument to a temperature of 160 °C, simulating hightemperature and high-pressure conditions.
Sand bed experiment.Prepare an OBDF slurry containing x% (x = 2, 4, 6) of the resin plugging agent and subject it to aging in an oven at 160 °C for 16 hours.Subsequently, weigh 350 cm 3 of quartz sand within a specied mesh size, thoroughly wash and dry it, then transfer it into the cylindrical chamber of a mediumpressure sand bed lter.Level and compact the sand uniformly before gently introducing 250 mL of the aged OBDF slurry enriched with a designated concentration of the plugging agent.Aer sealing the setup, apply pressure up to 0.7 MPa.Monitor the penetration and ltration of the bentonite slurry within the sand layer, and ascertain the depth of ltrate penetration into the sand bed aer a duration of 30 minutes.
Electrical stability test.According to, 26 the electrical stability (ES) of OBDF is an indicator of their emulsion stability and oil wettability.Therefore, measuring the ES values before and aer the addition of resin provides a direct assessment of the plugging agent's effect on the drilling uid's stability.The procedure involves stirring the prepared drilling uid for 15 minutes, then inserting the motor probe into the uid to obtain a reading.Given the intricate inuence of the drilling uid's chemical composition on its ES, it is essential to conduct multiple measurements of the drilling uid's ES and calculate the average value.
Rheological test.The rheological properties of resin were measured by Haake Mars 60 rheometer at 50 °C.
The rheological parameters of OBDF, such as the apparent viscosity (AV), plastic viscosity (PV), and yield point (YP) of OBDF, were determined through measurements of the viscosities at two shear rates of 600 rpm and 300 rpm at room temperature by using a ZNN-D6L rotational viscometer (Qingdao, China).Calculated from the values of Ø 600 and Ø 300 by the following formulas: 14 Apparemt viscosity (AV) = 0.5Ø 600 (mPa s) (2) where the Ø 600 and Ø 300 represent the reading on the rheometer dial at speeds of 600 and 300, respectively.
Characterizations Contact angle analysis.The contact angle of the resin was observed by an optical contact angle meter (KRUSS DSA30S, Germany).A smooth thin lm was obtained by pressing puried and dried samples for 5 min under 10 MPa at room temperature.Then, the contact angle between white oil and lm under air conditions was measured.
X-ray photoelectron spectrometry analysis.The elements of the sample were veried using X-ray photoelectron spectroscopy (XPS, Thermo Scientic ESCALAB 250Xi VG).
Fourier transform infrared spectroscopy (FT-IR) test.The oilabsorbing resin underwent analysis through Fourier transform infrared spectroscopy (FT-IR; model: Nicolet iS 10), encompassing 4 to 32 scans per millisecond across a scanning wavenumber spectrum of 4000 to 400 cm −1 .
TGA-DSC measurements.The thermal properties of the resin were determined using a thermal analysis apparatus (TGA/DSC 3+/1600 HT, METTLER TOLEDO) under nitrogen ow, with the temperature range set from 30 to 800 °C at a heating rate of 10 °C min −1 .Scanning electron microscopy analysis.Scanning electron microscopy (SEM) analyses were measured by a scanning electron microscope (Phenom ProX, thermos scientic).First, a specic quantity of resin was adhered to the glass slide to quantify the look of the material.The slide was then attached to the conductive adhesive aer being cured under vacuum for 30 minutes.Lastly, the resin was lightly coated with gold before being seen under a SEM.Scanning electron microscopy (SEM) analysis was conducted using a scanning electron microscope Paper RSC Advances (Phenom ProX, Thermo Scientic).Initially, a dened quantity of resin was adhered to a glass slide to facilitate the examination of the material's appearance.Subsequently, the slide was affixed to conductive adhesive following a 30 minutes vacuum curing process.Finally, the resin underwent a light gold sputtering to enhance its conductivity before SEM observation.
Crosslinking degree analysis.The crosslinking degree of the resin were veried using a eld nuclear magnetic resonance analyzer (VTMR20-010V-I, SuZhou).The resonance frequency is 22.00 MHz, the temperature of the magnet is controlled at 35.00 ± 0.01 °C, and the diameter of the probe coil is 10 mm.The XLD-3 inversion model was used to t the crosslinking density parameters.The T 2A (relaxation time of the signals from the cross-linked portion), T 2B (relaxation time of the signals from the uncrosslinked portion), and crosslinking density were obtained by testing.

Results and discussion
Synthetic mechanism of the oil-absorbing resin Fig. 1 presents the schematic representation of the synthetic mechanism for the oil-absorbing resin.In the ternary copolymer synthesis, methyl methacrylate functions as a hard monomer, octadecyl methacrylate serves as a exible macromolecule due to its long-chain ester composition, and styrene acts as a rigid molecule enhancing the resin's resistance to pressure.All three monomers contain C]C bonds, allowing them, under the inuence of an initiator and a cross-linking agent, to not only undergo bulk polymerization but also to create a three-dimensional network with a structured framework. 27,28Benzoyl peroxide facilitates polymerization by generating benzoyl radicals through thermal decomposition, and N,N 0 -methylenebisacrylamide (MBA) engages in cross-linking and polymerization via its dual C]C bonds.Furthermore, during the polymerization process, the aforementioned monomer free radicals contribute to the generation of additional chain free radicals, fostering the development of cross-linked networks. 29,30While the benzene ring in styrene reduces the polymer's crosslinking density, the long carbon chain in octadecyl methacrylate increases it.This combination achieves a balance between oil absorption capacity and polymer rigidity.Additionally, incorporating ethyl acetate creates a porous cross-linked structure on the resin's surface, which enhances its oil absorption capabilities. 31rmula screening of oil-absorbing resin Fig. 2 illustrates the oil absorption expansion rate of the resin under various conditions.The effects of dispersant PVA, crosslinker MBA, initiator BPO, and the ratio of MMA to SMA on the resin's oil absorption properties were investigated.Throughout this experiment, the total mass of monomers remained constant, with the amounts of PVA, MBA, and BPO specied as a percentage of their mass relative to the total monomer mass.
The presence of multiple side chain groups signicantly inuences the oil absorption capacity of resins.This phenomenon is attributed to compounds with multi-side chain structures expanding the internal volume of the resin via crosslinking agents.Consequently, when oil molecules penetrate the resin, the interaction surface between the resin and oil molecules enlarges, leading to an increased resin expansion rate.Thus, the proportions of MMA and SMA monomers utilized in the synthesis critically affect the resin's oil absorption expansion rate.Fig. 2(a) demonstrates that as the MMA : SMA molar ratio escalates, the oil-absorbing capability of the resin progressively improves.It is observed that an augmentation in MMA correlates with a heightened oil absorption rate.Furthermore, the oil absorption rate peaks when the MMA : SMA molar ratio reaches 2 : 1. 18 Below a 2 : 1 MMA : SMA molar ratio, the presence of SMA yields numerous cross-linking points within the resin.However, an excess of these points restricts the exibility of chain segments in the resin, diminishing the oil absorption expansion rate.Conversely, when the ratio exceeds 2 : 1, a reduced SMA concentration leads to a scarcity of long alkyl side chains in the polymer, thereby decreasing oil absorption capability.Fig. 2(b) illustrates the impact of varying monomer ratios on the resin's oil absorption capacity, revealing an initial increase followed by a decrease in oil absorption with  the rise in PVA concentration, peaking at a 2.5% PVA concentration.At low PVA concentrations, oil-soluble monomers experience inadequate dispersion in water, leading to monomer aggregation, increased impurities within the polymer, and a reduced oil absorption expansion rate.Conversely, excessively high concentrations of PVA result in over-dispersion of monomers, which adversely affects the conditions conducive to polymerization, thereby diminishing the oil absorption rate.
Fig. 2(c) depicts the curve correlating the content of crosslinker N,N 0 -methylenebisacrylamide with the resin's oil absorption capacity.At lower MBA concentrations, the polymer exhibits fewer cross-linking points, resulting in a less effective cross-linking network within the resin.This insufficiency was observed during experiments where a minimal cross-linking agent concentration resulted in a product that was turbid and challenging to purify.As illustrated in Fig. 2(c), a progressive increase in MBA concentration led to the formation of a densely cross-linked structure within the resin.This structure facilitates oil molecule ingress via capillary action, with increasing numbers of oil molecules causing cross-linking points to rupture, allowing the long alkyl side chains within the resin to extend and the resin to absorb oil and swell.Furthermore, a higher degree of cross-linking enhances the resin's oil storage capacity, thereby augmenting its expansion rate, as indicated by Zhang's research. 32The optimal oil absorption rate was observed at the MBA concentration of 1.5%; beyond this point, the oil absorption rate declined.This decrease is attributed to higher cross-linking agent concentrations resulting in shorter chain segments between cross-linking points, reducing their mobility and, consequently, the resin's swelling capability.
Fig. 2(d) illustrates the inuence of benzoyl peroxide (BPO) on the oil absorption characteristics of the resin, with the oil absorption expansion ratio peaking at a BPO concentration of 1.25%.Given the employment of oil-soluble monomers in the synthesis process, the oil-soluble free radical initiator BPO is utilized accordingly.BPO is recognized as an effective initiator for the synthesis of acrylic ester polymers. 33When the amount of BPO was low, the internal crosslinking degree of the resin was small, the contact area between oil molecules and polymers was also small, and the oil absorption rate decreased.The effect of initiators on the crosslinking degree of polymers had a peak.When there were too many initiators, not only would they not increase the oil absorption rate of the resin, but they would also affect the formation of polymer crosslinking structure and reduce the oil absorption rate of the resin.

Structure evaluation of oil-absorbing resin
The contact angle of the oil droplet on the resin lm surface under air condition was 28.1°shown in Fig. 3(a), and the contact angle of the water droplet on the resin lm surface under air condition was 39.26°shown in Fig. 3(b), indicating that the resin lm was both oleophilic and hydrophilic.On the one hand, there are benzene rings (from styrene) on the surface of the resin, and on the other hand, the long carbon chain in SMA also makes the resin exhibit lipophilicity.In addition, the hydrophilicity of the resin is due to the addition of hydrophilic crosslinking agent MBA, where the acrylamide group makes the resin hydrophilic.
The resin structure was investigated using XPS, and the results are shown in Fig. 4. As shown in Fig. 4(a), the resin contains three elements: C, N, and O, and the characteristic peak intensity of C element is the highest, while N element is the lowest.Fig. 4(b) shows the high-resolution XPS spectrum of C 1s of the resin.Aer deconvolution, four peaks can be found in the resin, the peaks with binding energy of 288.9 and 286.9 eV are C]O and C-O, indicating that the resin contains SMA and MMA.The peak with a binding energy of 285.9 eV is C 6 H 5 − , indicating the presence of St.In addition, the peak representing C]C was not found in the C 1s spectrum, indicating that the product does not contain reactive monomers.Fig. 4(c) shows the high-resolution XPS spectrum of N 1s of the resin.There were two peaks found in the resin.The peaks with binding energy of 402.irregular surface adorned with numerous gully-like structures.This uneven topology augments the contact surface between the resin and oil molecules, signicantly enhancing the oil absorption expansion rate.Fig. 5(b) provides an SEM image of the resin at 100 00× magnication, showcasing the presence of round microporous structures on the surface.These micropores facilitate the penetration of oil molecules into the resin's interior, further extending the contact surface between them, thereby augmenting the oil absorption expansion rate.
The polymer's molecular structure was examined using thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC).As depicted in Fig. 6(a), the resin's   weight reduction occurs in three distinct phases.The initial phase of weight loss extends from room temperature to 356 °C, during which the TGA curve maintains a nearly constant slope.The weight loss rate up to 356 °C is minimal, at only 1.5%, attributed to the volatilization of solvents in the resin.In the subsequent phase, spanning 356-401 °C, the resin's weight decreases signicantly, with the DTG (differential thermogravimetric) curve peaking at 380 °C, marking the onset of thermal degradation of the resins.At 401 °C, the residue is a mere 0.16%, indicating a high resin purity.Beyond 414 °C in the nal phase, the resin transitions to carbon, and the TGA curve stabilizes, demonstrating the resin's commendable thermal stability.These results align closely with the TGA ndings for nano-sized poly(styrene-lauryl acrylate), 34 showcasing even greater purity.In addition, the glass transition temperature (T g ) was characterized by DSC shown in Fig. 6(b).According to the curve, the soening point of resin is 115 °C.The glass transition temperature (T g ) refers to the temperature corresponding to the transition from a glassy state to a highly elastic state.Most of the cracks that cause leakage are located in deep formations, where the temperature is generally above 100 °C.Therefore, when the resin enters the formation exceeds the T g , the resin will transform into a high elastic state and enter the cracks due to the positive pressure difference.Aer oil absorption, the resin will expand to seal the leakage cracks.
The infrared (IR) spectroscopy results of the oil-absorbing resin are depicted in Fig. 7. Absorption peaks at 2923 and 2852 cm −1 correspond to the asymmetric and symmetric stretching vibrations of C-H in the methyl and methylene groups, respectively, alongside -CH. 18The peak at 1726 cm −1 is associated with the stretching vibration of the acrylate ester carbonyl (C]O), denoting the incorporation of MMA chain segments within the resin.Peaks at 1447 and 1383 cm −1 are ascribed to the C]C stretching vibrations in the benzene ring.Additionally, peaks at 806, 757, and 700 cm −1 are linked to the out-of-plane bending vibrations of hydrogen in the benzene ring, characteristic of monosubstituted benzene. 34The presence of styrene-acrylonitrile (SMA) chain segments is evidenced by peaks at 1187 and 1138 cm −1 , representing the C-O bond stretching vibrations, and a peak at 989 cm −1 , associated with the in-plane rocking vibration of -CH in the long-chain methylene group.Collectively, the IR spectroscopy results conrm the involvement of MMA, SMA, and styrene (St) in constructing the resin's skeletal structure.Furthermore, the absence of C]C absorption peaks in the range of 1640-1675 cm −1 suggests a comprehensive synthesis reaction, where the unsaturated groups of the three monomers have engaged in graing, leading to the formation of a three-dimensional network structure.
The results of the model rubber crosslinking density test are shown in Table 2.According to the result, the relaxation time of crosslinked signals is very short, while the relaxation time of the signals from the uncrosslinked portion is very long.The relaxation time represents the degree of freedom of hydrogen protons in the resin, indicating a high degree of resin crosslinking.In addition, the cross-linking density was 76.03 × 10 −4 mol mL −1 , indicating the formation of an internal crosslinking network structure in the resin.To a certain extent, the higher the cross-linking density of the resin, the more crosslinking bonds there are per unit volume of the resin, and the greater the degree of cross-linking.When oil molecules enter the interior of the resin, the cross-linking network is stretched by the oil molecules, causing the resin volume to increase and the oil absorption rate to increase.Table 3 presents the outcomes of the electrical stability and rheological tests.These tests evaluated the rheological properties and demulsication voltage of drilling uids before and aer the incorporation of 3% resin, under various temperatures and aging conditions.Relative to drilling uids devoid of resin, the apparent viscosity (AV), plastic viscosity (PV), and yield point (YP) exhibited increases both before and aer aging.This rise is attributed to the addition of solid resin particles, which heightens the ow friction resistance among solids and liquids, as well as between the solids and liquids themselves.Furthermore, the introduction of resin particles markedly elevates the demulsication voltage, a consequence of the enhanced viscosity of the drilling uid imparted by the resin particles, leading to a more stable emulsion and, hence, an increased demulsication voltage.Additionally, as illustrated in Table 3, the aging temperature's elevation signicantly affects the rheological properties and demulsication voltage of the drilling uid mud, suggesting commendable compatibility between the resin and mud.

Oil absorption performance of the resin
Effect of absorption time.Fig. 8 illustrates the resin swelling process over 24 h, segmented into four distinct stages.Initially Fig. 7 FTIR spectrum of the resin.Paper RSC Advances (0-6 h), the resin's swelling rate is notably swi, with a discernible slowdown in the swelling rate within the rst hour of resin and oil molecule interaction.Subsequently, aer this hour, there is a signicant acceleration in the swelling rate.This acceleration then diminishes in the second phase (6-15 h), culminating in a near cessation of swelling in the nal phase (15-24 h) as the resin approaches swelling equilibrium.Notably, the last 3 hours witness a reduction in swelling rate.In the initial stage of oil absorption, a fraction of oil molecules penetrate the interior of the resin molecules through capillary action, leading to a relatively slow oil absorption rate within the rst hour.Following this period, the oil molecules achieve comprehensive contact with the resin particles, transitioning the control of oil absorption from molecular diffusion to thermodynamics.At this period, sufficient oil molecules have entered the resin particles' interior, facilitated by van der Waals forces. 18The resin's exible macromolecular chain segments begin to expand, engaging in solvation with the oil molecules.This stage is marked by the highest oil absorption rate, driven by the resin's internal three-dimensional network composed of covalent bonding crosslinking points, physical crosslinking regions, and entanglement connections. 35As the exible macromolecular chains unfold, the majority of oil molecules inltrate the resin, slowing the diffusion rate of oil molecules within the resin.Consequently, during this third stage, the resin's oil absorption rate decelerates under dynamic control.
Ultimately, as the thermodynamic driving force and network elastic retraction force equilibrate, the resin achieves saturation swelling.Therefore, in the nal stage, the oil absorption rate of the resin essentially stabilizes.Effect of temperature.The resin was incorporated into nonwovens and encapsulated within a tube resistant to temperature and pressure, then subjected to conditions at room temperature and at 80, 120, 140, and 160 °C, respectively, immersed in 3# white oil.To determine the oil absorption swelling capability, the mass ratios before and aer oil absorption were evaluated using gel particles with a particle size of 0.1 mm.Fig. 9 displays the experimental data on the oil absorption expansion rate of the resin at room temperature, as well as at 80, 120, 140, and 160 °C.Notably, at 160 °C, the resin's oil   absorption expansion ratio reached 12.2.Research of the resin's oil absorption process at room temperature revealed that the resin's maximum oil absorption rate is governed by thermodynamics.Additionally, the resin's oil absorption saturation rate is primarily inuenced by its crosslinking density and the solvation interactions between oil molecules and the resin's oleophilic groups.Consequently, with rising temperatures, the initial stage of oil absorption, dominated by molecular diffusion, facilitates greater contact between oil molecules and the resin surface, thereby increasing the number of oil molecules penetrating the resin's interior.In the subsequent stage, dominated by thermodynamic control, the interaction between oil molecules and the resin's oleophilic groups becomes more comprehensive, enhancing the solvation effect and thus the resin's expansion volume.

Plugging performance of resin
Plugging experiment.The outcomes of the crack plugging experiment are detailed in Table 4. Without the addition of resin particles, OBDF exhibits inadequate sealing efficacy across three different crack sizes, resulting in total uid loss when the crack width reaches 0.75 mm.Conversely, the incorporation of 3% resin particles effectively seals small cracks and substantially mitigates leakage through slightly larger ssures.The plugging efficiency remains relatively consistent before and aer aging, underscoring the prepared resin particle plugging agent's robust stability.Furthermore, observations during the experiment revealed that the resin particles remained welldispersed within the mud, without any signs of agglomeration or clumping, demonstrating excellent compatibility between the resin particles and the drilling mud.Fig. 10 illustrates the state of the crack plate during the plugging experiment with a crack width of 0.25 mm.The regions highlighted in red represent resin particles.Observation reveals that resin particles accumulate within the crack.Subject to high temperature and pressure conditions, these particles undergo deformation and penetrate the crack under compressive forces.Owing to the resin's expansion and viscosity within the environment of oil molecules, it remains lodged within the crack, thereby effectively sealing the leak.

Sand bed experiment
Table 5 presents the results of sand bed experiments conducted with OBDF mud containing varying concentrations of resinbased plugging agents.The data indicate that as the concentration of resin particles is increased, the depth of intrusion into the sand bed signicantly diminishes.
Traditional plugging materials oen necessitate specic particle sizes to effectively seal lost circulation zones.In scenarios involving smaller void spaces, these rigid plugging materials must be crushed and resized before incorporation into the plugging mud.Conversely, the resin-based plugging materials developed in this study are capable of adapting to various pore sizes in loss channels, thanks to their inherent variability.Furthermore, the incorporation of styrene (St), with its benzene ring structure, imparts a degree of pressure resistance to the mixture.Beyond this, the resin particles exhibit a certain viscosity upon oil absorption, allowing them to persist within the leakage channels.This capacity enables them to form a pressure-sealing layer possessing adequate strength to mitigate uid loss effectively.

Sealing mechanism
Fig. 11 depicts a schematic representation of resin particles entering a leakage channel to seal a leak aer an underground well breach occurs.The resin particles, owing to their excellent deformability, navigate into the leakage channel.Accumulation of a substantial number of resin particles within the channel counterbalances the drilling uid column's pressure with the channel's internal pressure, thereby achieving effective ll. 36Over time, as the resin particles remain in the drilling uid, they absorb oil and swell, thereby fullling the leak plugging   objective.It is important to note that most plugging materials under study require favorable dispersibility in the oil phase, necessitating the synthesis of materials rich in lipophilic groups.However, the challenge of material retention within the leaking stratum is oen overlooked.This oversight stems from the fact that upon the entry of OBDF into the formation, the rock surface in contact with the drilling uid transitions to an oil-wet surface, which impedes the plugging material's adherence to the leaking wall. 37In response, our synthesis of the resin incorporates the water-based crosslinking agent MBA.Additionally, we minimize the use of styrene-acrylonitrile (SMA) with its lengthy carbon chains.To enhance the resin's pressure resistance and augment the addition of styrene (St), our ndings indicate that the resin material synthesized under these parameters exhibits superior plugging performance in such scenarios. 38

Conclusions
(1) A ternary self-swelling oil-absorbing resin was synthesized using polymeric monomers of St, SMA, and MMA.BPO served as the initiator, MBA as the cross-linker, and EAC as the poreforming agent.
(2) The molecular structure and characteristics of the resin were investigated using SEM, TG, and IR, providing insights into the resin's synthesis and oil absorption mechanism.This analysis further substantiated the process by which oil molecules inltrate the resin, resulting in its expansion.
(3) This study examines the impact of variations in MMA: SMA, MBA, PVA, and BPO ratios on the oil absorption capabilities of oil-absorbing resins to identify the composition that yields the highest oil absorption performance.
(4) The composite resin particles were utilized to plug fractures with widths ranging from 0.25 to 0.75 mm, demonstrating a ltration loss of 2.6 mL under high temperature and high pressure conditions in 0.75 mm cracks.Furthermore, the sealing mechanisms of the resin particles within geological formations were elucidated.
(5) The results of this study can be used in the eld of drilling uid plugging.The resin mentioned in this article can seal cracks of different widths, which can effectively solve the problem of difficult plugging in drilling operations.

Fig. 1
Fig.1Diagram of the synthetic mechanism of the oil-absorbing resin.

Fig. 2
Fig. 2 The oil absorption expansion rate of the resin in different conditions: (a) the effects of different monomer ratios on oil absorption of the resin; (b) the effect of PVA on absorption of the resin; (c) the effect of MBA on absorption of the resin; (d) the effect of BPO on absorption of the resin.
13 and 399.78 eV are N-(C]O)-and C-N-C, representing the presence of MBA.Fig. 4(d) shows the highresolution XPS spectrum of O 1s of the resin.Aer deconvolution, three peaks can be observed in the resin, representing C-O-C, O-(C]O)-C and O]C-N.C-O-C, O-(C]O)-C indicate the presence of SMA and MMA in the resin.The presence of O]C-N indicates the containment of crosslinking agent MBA in the resin.Fig. 5(a) displays scanning electron microscope (SEM) images of the resin at 1000× magnication, revealing an

Fig. 3
Fig. 3 The results of contact angle: (a) the contact angle of white oil on the resin film surface under air conditions; (b) the contact angle of water on the resin film surface under air conditions.

Fig. 4
Fig. 4 The results of XPS: (a) survey scan of the resin; (b) the highresolution XPS spectrum of C 1s of the resin; (c) the high-resolution XPS spectrum of N 1s of the resin; (d) the high-resolution XPS spectrum of O 1s of the resin.

Fig. 5
Fig. 5 SEM picture of the resin: (a) SEM pictures of the resin magnified 1000 times; (b) SEM pictures of the resin magnified 10 000 times.

Fig. 6
Fig. 6 TGA-DSC analysis of the resin: (a) TGA-DTG curve of the resin; (b) DSC curve of the resin.

Fig. 8
Fig. 8 Oil absorption performance of the high-oil-absorbing resin under different time conditions.

Fig. 9
Fig.9Curves for the oil absorption ratios of the resin at different temperatures.

Fig. 10
Fig.10The condition of the crack board in the plugging experiment of the crack width is 0.25 mm.

Table 3
The results of electrical stability test and rheological test

Table 4
The crack plugging experiment

Table 5
Sand bed experimental results of OBDF mud with different concentrations of resin